When a discount is added to a subscription via a coupon, a discount length is chosen. This can be either lifetime, or a fixed term with a specific end date. You can, however, override this discount length and choose to remove the discount at any time.

To do this, navigate to the individual subscription and then click on the Discounts tab. Here, you'll see any discounts applied to the subscription and you'll see a button on the right hand side that allows you to remove the subscription immediately.